Magnetic field sensors for use in measuring static (time-invariant) magnetic fields are known. Types of sensors include semiconductor Hall effect sensors, magnetoresistive sensors in magnetic materials, magnetic force microscopes, magnetostatic delay lines, and superconducting quantum interference device (SQUID) sensors. At low bandwidths (e.g., less than 10 kHz), Hall effect magnetic field sensors may be used to provide measurement with sensitivity on the order of 10-100 microteslas. Magnetoresistive thin film materials may be used to construct sensitive magnetic field sensors for detecting magnetic fields in the nanotesla range at room temperature (e.g., 300 K). Magnetoresistive sensors are described, for example, in U.S. Pat. No. 5,247,278, "Magnetic Field Sensing Device" and U.S. Pat. No. 5,500,590, "Apparatus for Sensing Magnetic Fields Using a Coupled Film Magnetoresistive Transducer". These sensors are based on anisotropic magnetoresistance (AMR) materials such as permalloy, which exhibit a 2-4% change in resistance per oersted (79.58 A/m) of magnetic field. Products such as the HMC 1001 from Honeywell produce 15 .mu.V output signals in response to 100 nT (nanotesla) magnetic fields. The output signal voltage of these magnetoresistive sensors are amplified to millivolt/volt signals using low-noise amplifiers. In general, the use of these sensitive magnetoresistive elements has been limited to detection of static or slowly varying magnetic fields. High speed sensing has been prevented by the limited bandwidth of the amplification stages and the large thermal noise voltage of the magnetoresistive sensing element.
Magnetic force microscopy, such as is described in U.S. Pat. No. 5,465,046, "Magnetic Force Microscopy Method and Apparatus to Detect and Image Currents in Integrated Circuits", relies on force exerted by a current of an integrated circuit on a miniature cantilevered magnetic tip. The intrinsic bandwidths are very low (e.g., less than 100 kHz) because they are limited by the mechanical resonant frequencies (e.g., 22 kHz) of the cantilever. The low damping rate of the mechanical resonance of the cantilever also limits the spatial scanning rate, and the site-to-site scanning rate is on the order of milliseconds.
Magnetostatic delay lines, such as are described in U.S. Pat. No. 4,926,116, "Wide Band Large Dynamic Range Current Sensor and Method of Current Detection Using Same", have high bandwidths, and generally have low spatial resolution. This is because a delay line generally has to be long (on the order of a few centimeters) for appreciable field sensitivities (e.g., a 1% change in delay for 1 oersted of magnetic field).
At present, superconductive quantum interference devices (SQUIDs) appear to have the best magnetic field sensitivity and the best field resolution (picotesla range) of devices known in the art. They have been used in a variety of scanning microscope applications for characterizing magnetic fields in microscopic structures. Although the sensitivity and noise properties of the SQUID microscopes are much better than magnetoresistive transducers, they require cryogenic operation and the SQUID loop cannot be brought in close proximity (for example, 10 .mu.m) to room temperature samples. The low temperature operation of the SQUIDs reduces the temperature of the sample and affects its performance. Since magnetic fields decay in a manner inversely proportional to the square of the distance between the structure under test and the SQUID sensing loop, this spacing requirement may result in significant loss of magnetic field resolution.
Magnetic field sensors may be used in characterization of complementary metal oxide semiconductor (CMOS) integrated circuits. Typically, CMOS integrated circuit power supply current is in the nanoampere range when there are no logic transitions and the clock is inactive. This low quiescent power supply current, known as I.sub.DDQ, is used to identify defects and reliability failure mechanisms. I.sub.DDQ increases the fault coverage of logic circuits by easily detecting simple stuck-at-faults (SAF), logically redundant SAFs and multiple SAFs. Conventional methods using scan registers require hundreds of patterns to detect the faulty responses in a complex microprocessor chip. The number of test vectors necessary to obtain fault coverage greater than 95% is very large. This significantly increases test and debugging time. I.sub.DDQ also permits detection of CMOS defects such as gate-oxide shorts, shifted transistor thresholds, bridges, and transmission gate faults.
As the channel lengths of CMOS transistors are scaled to the 50-100 nm regime, the voltage supply levels are being scaled down to 1-1.5 V to maintain the same channel electric fields and to lower power dissipation. To increase the current drive and speed, the threshold voltages are also being scaled down below 0.3 V. Low threshold voltages increase the transistor subthreshold leakage current dramatically (for example, 10-100.times. increase when CMOS channel lengths are scaled from 0.25 .mu.m to 0.1 .mu.m). This results in large currents at the supply pins, which may mask the increase in I.sub.DDQ currents due to defects and negate many of the advantages of I.sub.DDQ testing.